A black dog, described as "fine, handsome and valuable" was reported to "throw himself into the water" in an attempted suicide. His legs and feet were in "perfect stillness" – highly unusual for a dog in a river.

Even stranger, after he was dragged out he "again hastened to the water and tried to sink again…"

Eventually the handsome dog succeeded in his apparent suicidal mission and died.

He cited 21 apparent animal suicides, including a dolphin who let himself be captured

According to the Victorian press, this dog was far from alone in attempting such a fate. Shortly afterwards, two other cases appeared in the popular press: a duck drowned on purpose, and a cat hanged herself on a branch after her kittens had died.

It is not a new question: the ancient Greeks considered it too. Over 2,000 years ago, Aristotle cited a stallion that threw himself into an abyss after it became apparent that, like Oedipus, he had unknowingly mated with his own mother.

In the 2nd Century AD, the Greek scholar Claudius Aelian devoted an entire book to the subject. He cited 21 apparent animal suicides, including a dolphin who let himself be captured, several instances of hounds starving to death following the passing of their owners, and an eagle that "sacrificed itself by combustion on the pyre of its dead master".

A wild stag, rather than be overtaken by its pursuers, will fall into the jaws of an awful death

Just like the "handsome dog" that drowned, the idea of animal suicide remained a popular topic among 19th-Century Victorians. One psychiatrist, William Lauder Lindsay, attributed "suicidal melancholia" to animals of this disposition, describing how they could be "literally goaded into fury and mania" before a suicide.

At the time, these ideas were seized upon by animal rights groups, such as the UK-based Royal Society for the Prevention of Cruelty to Animals (RSPCA). Animal activists sought to humanise animal emotions, explains medical historian Duncan Wilson, of the University of Manchester, UK, who looked into historical cases of references to animal suicide in a 2014 research paper.

They did so, he says, to show animals "shared the capacity for self-reflection and intent, which included intent to kill themselves out of grief or rage.”

For example, in the 1875 edition of the RSPCA journal Animal World, the cover depicted a stag jumping to a suicidal death. The accompanying editorial stated that "a wild stag, rather than be overtaken by its pursuers, will fall into the jaws of an awful death".

However, as medicine advanced in the 20th Century the human attitude to suicide became more clinical, and these type of "heroic portrays" of suicidal animals dwindled.

Such stories should not fool us, according to Antonio Preti, a psychiatrist at the University of Cagliari in Italy, who scoured the scientific literature on animal suicide.

He looked at about 1,000 studies published over 40 years, and found no evidence of an animal knowingly attempting suicide in the wild. Cases such as those in Aelian's book are "anthropomorphic fables", he says.

Researchers now know that the mass deaths of lemmings are an unfortunate consequence of a dense population of creatures emigrating together at the same time.

In cases where a pet dies following its master's death, this can be explained by the disruption of a social tie, says Preti. The animal does not make a conscious decision to die; instead, the animal was so used to its master that it no longer accepts food from another individual.

"To think it died from suicide like a person after the death of a spouse is just a projection of a style of [romantic] human interpretation."

This example points to the one important fact. Stress can change the behaviour of an animal in a way that can threaten its life.

A video went viral of a wild-born orca in one of SeaWorld's parks that appeared to beach herself on the side of the tank for about ten minutes. Dozens of articles appeared stating she had attempted suicide.

The vast majority of cases are down to human intervention in some way

We know that orcas behave differently in captivity than in the wild, which is unsurprising given that a tank is a fraction of the size of the ocean. Unnatural environments have been known to stress captive orcas, triggering repetitive behaviours such as rubbing the side of their tanks and grinding their teeth.

When it comes down to it, says Barbara King of the College of William & Mary in Virginia, US, it is important to understand just how deeply these animals experience emotions. This can in turn inform us about what causes them to act in self-destructive ways.

"To my knowledge the vast majority of cases are down to human intervention in some way, whether the result of poaching or confinement," says King, who has written extensively about animal grief and suicide.

Many other animals kept in traumatic conditions also experience conditions similar to stress, post-traumatic stress disorder (PTSD) and depression.

It can be seen as a reflection of an animal trying to escape its imprisonment

A captive bear at a Chinese bear farm was reported to have smothered her son and then killed herself. This followed an extremely painful injection of a catheter into the bear cub's abdomen to extract bile, which is sometimes used in Chinese medicine. Newspaper reports suggested she had killed herself and her son to prevent more years of torture.

This is more likely another example of unnatural behaviour triggered by stress and being housed in an artificial environment for a prolonged period of time. It can also be seen as a reflection of "an animal trying to escape its imprisonment," says Preti.

Other animals often cited as attempting suicide are whales that strand in the wild.

It remains unclear why whales strand. One idea is that standings can be caused by a sick individual seeking the safety of shallower water. Because whales form social groups, others follow and strand also. This idea is now called the "sick leader hypothesis". But it is not considered suicide.

An even more subtle cause of what may appear to be self-destructive behaviour can also be easily explained. There are certain parasites that infect the minds of their hosts, causing mind-altering behaviours that help the parasites thrive. The host often dies in the process.

For example, the parasite Toxoplasma gondii infects mice and switches off their innate fear of cats. If the cat eats the mouse, the parasite reproduces. A 2013 study found that a T. gondii infection permanently deletes this fear, even after the parasite has been exterminated.

Similarly, a parasitic fungus called Ophiocordyceps unilateralis can control the minds of ants, turning them into zombies. It steers the insects to their deaths in locations where the fungus can thrive.

Lastly, mother spiders let their young eat them. Though they die in the process, this sacrifice is not suicide, but rather an extreme act of parental care. The mother spiders provide their own bodies as a vital first nutritious meal that ensures their offspring's survival.

To say such behaviour does not constitute suicide requires a definition of what does. Suicide is commonly defined as "the action of killing oneself intentionally".

We know that some animals kill themselves. The question is whether they intend to. The mother spider, for example, may behave this way to primarily provide food, not die. The mother bear may have acted unnaturally due to stress, not with the intent of killing her and her cub.

Planning a suicide requires a detailed understanding about our place in the world

Some experts believe this question is impossible to answer.

Just as we have long underestimated animal cognition, we cannot yet read an animal's mind. "I'm not convinced [animal suicide] is a question science can answer," says King. "We can look at their visible behaviour as we do with grief, but we cannot look at what amounts to harm that comes to an animal and assess whether it's intended or not intended."

But others disagree. They say some people intend to kill themselves, while animals do not, due to differences in cognitive ability. The key difference, they say, is our ability to think far into the future.

Planning a suicide requires a detailed understanding about our place in the world, and an ability to imagine ourselves as no longer present. This requires imagination.

"Humans have a capacity to imagine scenarios, reflect on them, and embed them into larger narratives," says Thomas Suddendorf, an evolutionary psychologist at the University of Queensland in Australia.

If animals denied the risks of death as many humans do, zebras might knowingly graze near lions

"There appears to be something fundamentally distinct about human mental time travel when compared to the capacities of our closest-surviving animal relatives."

This ability comes at a price. "We worry about many things we can do little about, and we can experience persistent anxiety about things that may never eventuate," says Suddendorf.

Most of us overcome these worries. We have an in-built optimism bias, which gives us a rosier view of the future, but this is not the case for those with depression, for whom the future often appears very bleak.

"Clinical psychologists are beginning to recognise and disentangle the important roles aspects of foresight play in our mental health," says Suddendorf.

We are the only animal able to understand and cope with our own mortality

"One of the realities is that you are going to die." The rest of us have an amazing ability to ignore this eventuality, which Varki dubs "an evolutionary quirk".

"We need that denial," he says. "Otherwise we might curl up and do nothing." Instead, some us engage in reckless activities such as climbing dangerous mountains, driving cars too fast and taking mind-altering drugs.

Varki therefore proposes that all cases of apparent animal suicide can be explained by other means.

"It's a fear of dangerous situations that potentially lead to death," says Varki.

Thinking of it this way makes more sense. If animals denied the risks of death as many humans do, zebras might knowingly graze near lions, fish swim alongside crocodiles, and mice stare into the eyes of snakes.

If they were self-aware to the extent we are, they might also stop defending their territories or foraging for food. They have an appropriate in-built response to fear for good reason: it keeps them alive.

We are the only animal able to understand and cope with our own mortality, Varki argues, precisely because we are such optimistic creatures with a sophisticated level of self-awareness.

"What is suicide?" asks Varki. "It is inducing your own mortality, but how can you induce it if you don't know you have mortality? It's [therefore] quite logical that suicide should be uniquely human."

Melissa Hogenboom is BBC Earth's feature writer. She is @melissasuzanneh on Twitter.

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